1. Field of the Invention
An aspect of the present invention relates to at least one of a method for manufacturing an electrochromic display device and an electrochromic display device.
2. Description of the Related Art
In recent years, attention to an electronic paper has been attracted as an electronic medium that replaces paper. An electronic paper is characterized in that it is possible to use a display device like a paper, and a characteristic different from that of a conventional display device such as a Cathode Ray Tube (CRT) or a liquid crystal display is required. For such an electronic paper, there is a reflection-type display device with low electric power consumption and a less amount of visual fatigue as well as a self-emitting display device such as an organic EL display.
For a reflection-type display device, there is, for example, a reflection-type liquid crystal display device using a cholesteric liquid crystal, but such a display device uses a selective light reflection and has a large number of substrates for forming the display device, so that a reflectivity, a contrast ratio, a color saturation, or a color reproduction range is not sufficient and visibility is far inferior to that of a paper.
Meanwhile, for a reflection-type display device, there is an electrochromic display device using an organic electrochromic material with both high color reproducibility and display memory property, etc. An electrochromic phenomenon is also referred to as “electrochromism”, and refers to a phenomenon in which a reversible oxidation reduction reaction is caused by applying an electric voltage thereto to change a color reversibly. An electrochromic display device is a display device utilizing color appearance/color disappearance (which will be described as coloring/discoloring, below) of an electrochromic compound that causes such electrochromism. Such an electrochromic display device has features of being a reflection-type display device, having a memory effect, being able to be driven at a low voltage, etc., and hence, is a potent candidate of a display device technique used for an electronic paper application.
It is possible for an electrochromic display device to develop various colors depending on a structure of an electrochromic compound, and hence, it is possible to provide a multicolor display. Japanese Patent Application Publication No. 2009-163005 and Japanese Patent Application Publication No. 2010-033016 disclose electrochromic display devices with a structure in which a plurality of layers of display electrodes and a coloring layer(s) formed of an electrochromic compound are laminated on one display substrate.
Japanese Patent Application Publication No. 2009-163005 discloses an electrochromic display device with a structure in which a display electrode is formed of an indium tin oxide (ITO) fine particle(s). Furthermore, Japanese Patent Application Publication No. 2010-033016 discloses an electrochromic display device having a display electrode and insulating layer that have permeability into an electrolyte, wherein an ITO film that is film-formed by a sputtering method is used as a display electrode. Furthermore, Japanese Patent Application Publication No. 2010-282128 discloses an electrochromic display device formed by holding an electrochromic layer on a porous sheet body having an electrical conductivity and superposing a plurality of porous sheets thereon.
However, an electrically conductive film of an electrochromic display device as disclosed in Japanese Patent Application Publication No. 2009-163005 is formed by an ITO fine particle(s), wherein an electrical conductivity of an electrically conductive film formed by an ITO fine particle(s) is low and a volume resistivity thereof is 2-5 digits higher as compared with that of an ITO electrically conductive film formed by a vacuum film formation such as sputtering. Thus, a volume resistivity and electrical conductivity of an electrically conductive film that forms a display electrode layer are high and low, respectively, which are causes that produce an in-plane non-uniformity in a display image as a surface area of such a display electrode is increased. Furthermore, when a thickness of a display electrode layer formed of an ITO fine particle(s) is increased to improve an electrical conductivity of an electrically conductive film, degradation of reflectivity and degradation of quality of an image to be displayed are caused, and hence, are not preferable.
Furthermore, an ITO film that has been film-formed by sputtering is used for a display electrode of an electrochromic display device disclosed in Japanese Patent Application Publication No. 2010-033016, wherein such an ITO film that is film-formed by sputtering is dense and has a poor ion permeability, and application of a large voltage is needed for a coloring and discoloring reaction of an electrochromic layer. That is, in a structure in which a plurality of display electrodes and electrochromic layers are laminated, increasing of a voltage to be applied to attain coloring and discoloring of an electrochromic layer provided separately from an opposed substrate is needed.
Furthermore, in an electrochromic display device as described in Japanese Patent Application Publication No. 2010-282128, blur that originates from an increase of a distance from an opposed electrode to each display electrode due to a thickness of a porous sheet per se may be generated in an image to be displayed, and moreover, an increase of a driving voltage may be required.